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You can purchase suspension-corrected rigid forks which are designed to work with the geometry that suspended frames offer (Essentially the axle-crown measurement places the head tube where it needs to be. Pretty simple.) There are a few manufacturers out there, but my favorite is Surly (in terms of quality and in value. They offer several forks for 26"-29" with disc and canti options.)

While installing a non-suspension-corrected fork might not totally ruin the handling of a particular bicycle, it would be VERY noticeable (as 120 mm is a good bit of distance to be dropping the front end of a bike.) It will be worth the investment.

Even a suspension corrected corrected fork will feel and handle differently than a fork with suspension. The Surly forks that I've seen are suspension corrected for 80mm. 40mm is still a noticeable difference in the front of the bike, but it's something that you can adjust to pretty quickly.
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jimirings♦Jan 20 '13 at 16:24

The 26" Troll fork is corrected for 100mm, but most of their other 29" forks are corrected for 80mm.
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WTHarperJan 20 '13 at 17:05